One type of conventional magnetoresistive sensor, often called a “spin-valve” (SV) sensor, has a stack of layers that include two ferromagnetic layers separated by a nonmagnetic spacer layer. One ferromagnetic layer has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and the other ferromagnetic layer has its magnetization direction “free” to rotate in the presence of an external magnetic field. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the fixed-layer magnetization is detectable as a change in electrical resistance.
The SV magnetoresistive sensor used in all current magnetic recording hard disk drives operates with the sense current directed parallel to the planes of the layers in the sensor layer stack, so it is referred to as a current-in-the-plane (CIP) sensor. In a disk drive CIP-SV read sensor or head, the magnetization of the fixed or pinned layer is generally perpendicular to the plane of the disk, and the magnetization of the free layer is generally parallel to the plane of the disk in the absence of an external magnetic field. When exposed to an external magnetic field from the recorded data on the disk, the free-layer magnetization will rotate, causing a change in electrical resistance.
A SV type of magnetoresistive sensor has been proposed that operates with sense current perpendicular to the planes (CPP) of the layers in the sensor stack. CPP-SV read heads are described by A. Tanaka et al., “Spin-valve heads in the current-perpendicular-to-plane mode for ultrahigh-density recording”, IEEE TRANSACTIONS ON MAGNETICS, 38 (1): 84–88 Part 1 January 2002. Another type of CPP sensor is a magnetic tunnel junction (MTJ) sensor in which the nonmagnetic spacer layer is a very thin nonmagnetic tunnel barrier layer. In a MTJ sensor the tunneling current perpendicularly through the layers depends on the relative orientation of the magnetizations in the two ferromagnetic layers. While in a MTJ magnetoresistive read head the spacer layer is electrically insulating and is typically alumina (Al2O3), in a CPP-SV magnetoresistive read head the spacer layer is electrically conductive and is typically copper.
For maximum read-head stability and response-linearity without hysteresis in all CIP-SV, CPP-SV and MTJ read heads, the magnetization of the free layer should be maintained in a saturated single domain state in the absence of an external magnetic field. In such a state, the local magnetization everywhere in the free layer, including the ends or side edges, is essentially “longitudinal”, i.e., along the length of the free layer and the cross-track direction of the head and parallel to the plane of the magnetic recording medium. Ferromagnetic biasing layers are typically used to achieve longitudinal biasing of the free layer. U.S. Pat. No. 6,023,395 describes an MTJ magnetoresistive read head that has a biasing ferromagnetic layer located in the sensor stack and magnetostatically coupled across a spacer layer with the free layer. U.S. Pat. No. 6,473,279 also describes CPP sensors with longitudinal biasing layers located in the sensor stack.
One limitation with in-stack biasing in CPP magnetoresistive sensors is that all of the layers making up the biasing structure, i.e., the biasing layer, a spacer layer, and an antiferromagnetic layer if the biasing layer is to be exchange-coupled, must be electrically conductive and add very little resistance to the sensor stack. Also, the requirement of a second antiferromagnetic layer in the sensor to exchange-couple the biasing layer requires a second annealing step in the presence of an applied field to set the magnetization direction of the biasing layer since the magnetization directions of the biasing layer and the pinned layer are orthogonal.
What is needed is a CPP magnetoresistive sensor with improved in-stack biasing of the sensor free layer.